The present invention relates to fluid controllers of the type used to control the flow of fluid from a source of pressurized fluid, such as a vehicle power steering pump, to a fluid pressure operated device, such as a vehicle steering cylinder.
In particular, the present invention relates to fluid controllers of the xe2x80x9copen-centerxe2x80x9d type, i.e., those in which the valving, when it is in its neutral position, defines an open flow path from the inlet port to a port connected to a downstream fluid pressure operated auxiliary device. Thus, fluid controllers of this type are referred to as having xe2x80x9cpower beyondxe2x80x9d capability, because they are able to communicate hydraulic power to another device on the vehicle which is downstream (xe2x80x9cbeyondxe2x80x9d), relative to the SCU.
In a fluid controller of the open-center type, the valving typically defines a power beyond flow path from the inlet port to the power beyond port, this power beyond path including a neutral variable orifice (AN) which has its largest flow area when the valving is in neutral, and a decreasing flow area as the valving is displaced from neutral. In addition, the controller valving defines a main fluid path communicating from the inlet port to a control (cylinder) port, the main fluid path including a main flow control (A1) orifice. The A1 orifice begins to open after several degrees of displacement (deflection) of the SCU valving, with the orifice area typically increasing with further valve deflection. In some open-center type units, the A1 orifice is always open, thus effectively comprising a xe2x80x9cfixedxe2x80x9d orifice, and it will be understood that references hereinafter, and in the appended claims, to a xe2x80x9cfirst flow control orificexe2x80x9d can mean and include either a variable or a fixed A1 orifice.
As is well know to those skilled in the art of fluid controllers of the open-center type, pressure begins to build in the main fluid path as the neutral variable orifice (AN) in the power beyond path gradually decreases in flow area, i.e., the power beyond path may be viewed somewhat as a xe2x80x9cleakxe2x80x9d in the main fluid path which needs to be closed off. Therefore, it has been common practice in such open-center, power beyond SCUs to completely close off the neutral orifice in the power beyond fluid path at a valve displacement somewhat less than the maximum valve displacement, in order to be able to generate as much steering pressure as possible when rotating the steering wheel at a maximum rate of rotation (which inherently results in maximum valve displacement).
Although such open-center, power beyond fluid controllers have been quite successful commercially, there has been one disadvantage inherent in such vehicle hydraulic systems. By way of example, on a commercial mower having an open-center SCU, and wherein the mower decks are raised hydraulically and constitute the xe2x80x9cpower beyondxe2x80x9d hydraulic function, the overall vehicle system performance has been considered very acceptable. However, when the vehicle is being maneuvered in a series of tight turns, with the mower decks raised, if the vehicle operator rotates the steering wheel at a rate sufficient to displace the valving to its maximum displacement, the power beyond fluid path closes somewhat before the SCU valving reaches its maximum displacement. In the situation described, without pressurized flow from the power beyond port, the mower decks may move from their raised position to a lowered position at a time when such is not desirable. Thus, references hereinafter, and in the appended claims, to an auxiliary device xe2x80x9crequiring a predetermined minimum flow availabilityxe2x80x9d will be understood to mean and include situations of the type described above, in which is it is simply not desirable, from the viewpoint of either the vehicle OEM or the operator, for the auxiliary device to have pressurized flow thereto from the auxiliary port cut off when the SCU valving is in its maximum displacement position.
The problem described above has led the assignee of the present invention to include, for many years now, in its product brochure for open-center power beyond SCUs, a statement advising customers to avoid using auxiliary (power beyond) functions that require constant flow while the vehicle is being steered. This has been considered unfortunate because, for many vehicle applications, the potential customer may not be able to use a particular power beyond function in combination with an open-center SCU, even though such a combination would otherwise be ideal or at least, very desirable. Alternatively, the customer may decide to use such a combination anyway, but at the expense of periodically having the power beyond device lose power. Clearly, neither alternative is fully acceptable.
Accordingly, it is an object of the present invention to provide an improved fluid controller of the open-center, power beyond type which is capable of overcoming the above-described disadvantages of the prior art device.
It is a more specific object of the present invention to provide such an improved fluid controller in which it is possible to maintain control over the downstream, auxiliary hydraulic devices even when the SCU is being operated at maximum valve displacement.
The above and other objects of the invention are accomplished by the provision of an open-center fluid controller operable to control the flow of fluid from a source of pressurized fluid to a fluid pressure operated priority device and to a downstream fluid pressure operated auxiliary device requiring a predetermined minimum flow availability. The fluid controller includes a housing defining an inlet port for connection to the source of pressurized fluid, a control port for connection to the priority device, and an auxiliary port for connection to the auxiliary device. Valving is disposed in the housing of the controller and defines a neutral position, a normal operating position, and a maximum displacement position. The housing and the valving cooperate to define an auxiliary fluid path providing fluid communication from the inlet port to the auxiliary port and including a neutral variable orifice, having a maximum flow area when the valving is in the neutral position, and a decreasing flow area as the valving is displaced, through the normal operating position, toward the maximum displacement position. The housing and the valving further cooperate to define a main fluid path providing fluid communication from the inlet port to the control port and including a first flow control orifice.
The improved open-center fluid controller is characterized by the valving defining an auxiliary flow control orifice providing fluid communication from the inlet port to the auxiliary port when the valving approaches and is in the maximum displacement position. The auxiliary flow control orifice has a flow area operable to permit the predetermined minimum flow availability to the auxiliary device.